Conventional active tip clearance control (ATCC) systems in a turbofan gas turbine engine use the fan driven bypass air, a portion of which is directed through a diffusion duct to the high pressure turbine case manifold. This portion of bypass air is directed to flow over the high pressure turbine case through a series of impingement holes. A modulating valve may be incorporated between an air inlet and the diffusion duct. This valve adjusts the portion of bypass air flow according to the engine requirements such that the appropriate tip clearance between the turbine blades and the turbine shroud is maintained. Conventional ATCC systems rely on the pressure differential between an inlet scoop at the bypass duct and a downstream location of the bypass duct where the vent cooling air is dumped. For engines in which this pressure differential is very low, the operation and/or efficiency of the ATCC system may be at risk.
Therefore, there is a need for improvements to an ATCC system.